π-Expansion-Directed Modulation of Spin–Fluorescence Coupling in Spin-Crossover Hofmann-Type Frameworks

High Resolution Image Download MS PowerPoint Slide The coupling between spin crossover (SCO) and luminescence provides a direct, noninvasive optical readout of spin-state switching, which is pivotal for the development of advanced molecular sensors, memory devices, and opto-spintronic applications. Herein, we report two two-dimensional Hofmann-type coordination polymers, i.e., {Fe 2 (PYNA) 4 [Ag(CN) 2 ] 4 } ( 1 ) and {Fe 3 (PYAN) 6 [Ag(CN) 2 ] 6 }· o -DCB ( 2, o -DCB = ortho -dichlorobenzene), based on two extended π-conjugated ligands, 4-(2-naphthalenyl)pyridine (PYNA) and 4-(2-anthracenyl)pyridine (PYAN), respectively. Divergent SCO behaviors were observed for the two compounds, as confirmed by temperature-dependent magnetic susceptibility measurements, structural analyses, and differential scanning calorimetry measurements. In addition, light-induced excited spin-state trapping effects were found in the two SCO compounds, and bidirectional photoswitching of spin states can be realized reversibly. Crucially, a comparative variable-temperature fluorescence (FL) study highlights a π-expansion-driven contrast: SCO-FL coupling is operative in 1 but absent in 2 . Theoretical calculations directly account for the distinct energy transfer pathways in 1 and 2, rationalizing their contrasting outcomes in achieving SCO-FL coupling. This study provides important support for the precise modulation of the spin-optical coupling properties through ligand engineering.